Get 8259 interrupt vector mappings

        Entry:
          AX = DE0Ah

        Return:
          AH = 00h successful
               BX = first vector used by master 8259 (IRQ0)
               CX = first vector used by slave 8259 (IRQ8)
          AH nonzero: failed

        Note:   CX is undefined in systems without a slave 8259